A method of fabricating a solar cell according to the OECO (Obliquely Evaporated Contact) process is disclosed for example in Renewable Energy, Vol. 14, p.83 (1998). The OECO process is a method of fabricating solar cells proposed by R. Hezel et al. of Institut für Solarenergieforschung GmbH Ha meln/Emmerthal (ISFH), Germany. A representative structure of a light-receiving surface of the OECO solar cell is schematically shown in FIG. 2 (a solar cell fabricated by the OECO process may occasionally be referred to as OECO solar cell, hereinafter). The OECO solar cell is configured so that a plurality of parallel grooves are formed on the main surface of a silicon single crystal substrate, which will serve as the light-receiving surface 3 later, and so that electrodes 6 for extracting output are formed on the inner side faces of the individual grooves on a single side as viewed along the width-wise direction of the grooves. This constitution successfully reduced the shadowing loss of the solar cell to as small as approx. 5% of the total light-receiving area. Because a typical solar cell having the electrodes formed by the screen printing method generally suffers from a shadowing loss of as large as approx. 12%, it is understood that the OECO solar cell has a sharply reduced shadowing loss, and that a large energy conversion efficiency is attainable.
In recent years, there is a strong demand for cost reduction in fabrication of the solar cells. More specifically, thinning of solar cells can reduce the amount of single crystal silicon per unit area used for the solar cells, and can reduce the cost to some extent. Thinning of the OECO solar cell, which requires a large number of grooves to be formed on the main surface, however undesirably tends to reduce the mechanical strength.
Besides those described in the above, there are known various solar cells having modified shapes of the electrodes formed on the light-receiving surface or back surface of the cell in order to improve the conversion efficiency. One publicly-known example of the solar cell is such as having grooves or bottomed holes for forming electrode contact mechanically carved or bored in the semiconductor single crystal substrate, and having metal for composing the electrodes filled in the grooves or bottomed holes. This type of cell was presented by two research groups at the 28th IEEE Photovoltaic Specialists Conference held in Anchorage in 2000.
The method by which the groove portions for forming electrode contact of the solar cell are mechanically carved, and the method by which the bottomed holes for forming electrode contact are mechanically bored were proposed independently by Institut für Solarenergieforschung GmbH Hameln/Emmerthal, Germany, and Fraunhofer Institute for Solar Energy Systems ISE, Germany, respectively. Specific procedures for carving the groove portions for forming electrode contact are such as follows. First, a plurality of nearly-parallel groove portions for forming electrode contact are mechanically carved on a semiconductor single crystal substrate (e.g., silicon single crystal substrate) having an insulating film such as a silicon oxide film (or silicon nitride film) formed thereon. Depth of the groove portions is set to 5 to 50 μm, and width thereof to several-hundred micrometers or around. The groove portions can be carved by scanning once or several times over the substrate with a high-speed rotary blade having several hundreds to thousands cutting edges. After the carving of the groove portions, a metal is uniformly deposited on the main surface to thereby form an electrode layer.
It is also possible to form bottomed holes for forming the electrode contact so as to be linearly aligned at regular intervals. Depth of the bottomed holes herein is again set to 5 to 50 μm similarly to the case where the groove portions are formed, and diameter of the opening of the bottomed holes is set to several-hundred micrometers or around. This type of bottomed holes can be bored by irradiating predetermined sites with KrF excimer laser, Nd:YAG laser or the like.
Thus-fabricated solar cells, being passivated with the insulating film in the non-contact area of the surface thereof, are advantageous in suppressing surface recombination of photo-generated carriers, and in consequently raising the conversion efficiency of the solar cells. This process is also advantageous in that the groove portions and bottomed holes for forming electrode contact can be formed in a relatively simple manner because formation thereof needs no photolithographic technique.
On the other hand, strong demands focused on the solar cells at present are improvement in the energy conversion efficiency and cost reduction. Among others, the cost reduction can be realized by thinning of the solar cells to thereby reduce the amount of silicon single crystal substrate used for the cells. Thinning of the semiconductor single crystal substrate, however, undesirably lower the mechanical strength of the resultant solar cells. This inventors further revealed that formation of the electrodes by carving or boring the groove portions or bottomed holes in the semiconductor single crystal substrate inevitably causes damages to the substrate per se, and this may further degrade the mechanical strength.
It is therefore a subject of this invention to provide a solar cell excellent in the mechanical strength, and a method of fabricating the solar cell.